(a) Field of the Invention
The present invention relates to a display device. More specifically, the present invention relates to an organic electroluminescent (EL) display, a display panel, and a driving method thereof.
(b) Description of the Related Art
In general, an organic electroluminescent (EL) display is a display device that electrically excites a phosphorous organic compound in a plurality of organic light emitting diodes (OLEDs) to emit light. The organic EL display voltage- or current-drives N×M organic emitting cells to display images. An organic emitting cell of the organic EL display includes an anode (ITO), an organic thin film, and a cathode layer (metal). The organic thin film has a multi-layer structure including an emitting layer (EML), an electron transport layer (ETL), and an hole transport layer (HTL) for maintaining balance between electrons and holes and improving emitting efficiencies, and it further includes an electron injecting layer (EIL) and an hole injecting layer (HIL).
Methods for driving the organic emitting cells include the passive matrix method, and the active matrix method using thin film transistors (TFTs) or MOSFETs. The passive matrix method forms cathodes and anodes to cross (or cross over) with (or perpendicular to) each other, and selects lines to drive the organic emitting cells. The active matrix method connects a TFT and a capacitor with each indium tin oxide (ITO) pixel electrode to thereby maintain a predetermined voltage according to a capacitance of the capacitor. The active matrix method can further be classified as a voltage programming method or a current programming method according to signal forms supplied for maintaining a voltage at a capacitor.
FIG. 1 shows a conventional pixel circuit for driving an organic EL element using TFTs and representatively illustrates a pixel circuit coupled to a data line Dm and a scan line Sn from among N×M pixel circuits (or cells). As shown, a driving transistor M1 is coupled to an organic EL element OLED to supply a current for light emission thereto. The current of the driving transistor M1 is controlled by a data voltage applied through a switching transistor M2. A capacitor Cst (or a storage capacitor) for maintaining the applied voltage for a predetermined time is coupled between a source and a gate of the driving transistor M1. A gate of the transistor M2 is coupled to a scan line Sn, and a source thereof is coupled to a data line Dm.
In operation, when the transistor M2 is turned on by a select signal applied to the gate of the transistor M2, a data voltage is applied to the gate of the transistor M1 through the data line Dm, and the current flows to the organic EL element OLED through the transistor M1 in correspondence to the data voltage applied to the gate of the transistor M1 to thus generate light emission.
The current flowing to the organic EL element OLED in this instance is given in Equation 1.
                                                                        I                OLED                            =                                                β                  2                                ⁢                                                      (                                          Vgs                      -                      Vth                                        )                                    2                                                                                                        =                                                β                  2                                ⁢                                                      (                                          VDD                      -                      Vdata                      -                                                                      Vth                                                                                      )                                    2                                                                                        Equation        ⁢                                  ⁢        1            where IOLED is a current flowing to the organic EL element OLED, Vgs is a voltage between the gate and the source of the transistor M1, Vth is a threshold voltage of the transistor M1, Vdata is a data voltage, and β is a constant.
As given in Equation 1, a current corresponding to the applied data voltage (Vdata) is supplied to the organic EL element OLED, and the organic EL element OLED then emits light in correspondence to the supplied current in the pixel circuit of FIG. 1.
In addition, a voltage (VDD) supply line for supplying the voltage of VDD to the pixel circuit is shown in FIG. 1 as a horizontal line or a vertical line. Referring now to FIG. 2, when multiple transistors are driven, the voltage (VDD) supply line applied to the pixel circuit can be represented as a horizontal line. In the case of FIG. 2, loads (impedance) at the transistors are increased, a large amount of currents are spent, and a voltage drop is generated between a voltage supply point of a first transistor of an input terminal and a voltage supply point of a transistor of a last terminal. As such, the voltage of VDD applied to a right pixel circuit 20 of the voltage (VDD) supply line is lower than the voltage of VDD applied to a left pixel circuit 25, and a long range (LR) uniformity problem is generated in FIG. 2. The voltage drop problem of the voltage (VDD) supply line is varied depending on design conditions to which the input of the voltage (VDD) supply line is coupled.
Also, a short range (SR) uniformity problem is generated because the amount of currents supplied to the organic EL element OLED is varied by a deviation of the threshold voltage (Vth) of a thin-film transistor (TFT) caused by non-uniformity of the manufacturing process, in addition to a brightness difference generated by a voltage drop of the above-described voltage (VDD) supply line.
To solve the problems, FIG. 3 shows a pixel circuit for preventing non-uniformity of brightness caused by variation of the threshold voltage (Vth) at the driving transistor M1, and FIG. 4 shows a drive timing diagram for driving the circuit of FIG. 3.
It is needed in the circuit of FIGS. 3 and 4 for a data voltage for driving a driving transistor to correspond to the voltage of VDD while a control signal of a signal line AZn is at a low-level. Further, when the control signal of the signal line AZn is at a high-level and a low-level data voltage is applied to a data line Dm, the voltage between a gate and a source of a driving transistor M1 is given in Equation 2.
                    Vgs        =                  Vth          -                                    C1                              C1                +                C2                                      ⁢                          (                              VDD                -                Vdata                            )                                                          Equation        ⁢                                  ⁢        2            where Vth is a threshold voltage at the transistor M1, Vdata is a data voltage, and VDD is a power supply voltage. However, since the data voltage is divided by capacitors (or capacitances) C1 and C2 as is shown from Equation 2, the pixel circuit of FIG. 3 is restricted in that it must either have a high data voltage (Vdata) or a high capacitance at the capacitor C1 to compensate for the capacitances at the capacitors C1 and C2.